Formula One championship leader Nico Rosberg and Mercedes team mate and title rival Lewis Hamilton limbered up for their U.S. Grand Prix duel with each setting the pace in practice sessions.
German Rosberg, chasing his 10th win of the season and 33 points clear of the reigning triple world champion, ended the day with the fastest time after lapping the Circuit of the Americas in one minute 37.358 seconds.
Hamilton, winner in three of the last four races at the Texan track including 2015 when he took his third title, was quickest in the morning in 1:37.428 with Rosberg second and 0.315 slower.
Briton Hamilton dropped to third in the afternoon, 0.291 behind Rosberg, with Red Bull's Australian Daniel Ricciardo splitting the dominant Mercedes pair.
Rosberg and Hamilton are the only drivers still in contention for the title with four races remaining while Mercedes have already clinched the constructors' crown for the third year in a row.
Hamilton needs another victory on Sunday but knows that even winning all the remaining races will not be enough if Rosberg finishes them in the runner-up position, and the German has said he is not about to settle for second best.
Red Bull's Dutch teenager Max Verstappen was third fastest in the morning with Ferrari's Kimi Raikkonen fourth and Force India's Nico Hulkenberg fifth.
Ferrari's Sebastian Vettel, like Hamilton a past winner in Austin, was eighth in the opening session after hitting a kerb, smashing bodywork, and driving back to the pits one-handed while holding the remnants of his right side mirror in the other.
The German came back strongly after lunch and was fourth, with Verstappen fifth.
Britain's Jordan King made his Formula One practice debut with Manor Racing, setting the 20th fastest time out of 22, while Germany's Pascal Wehrlein sitting out the session before returning in the afternoon.
Three drivers - Hulkenberg, Manor's Esteban Ocon and Williams' Valtteri Bottas - tried out the new 'halo' head protection device on their cars early in the session.
At the track, Formula One supremo Bernie Ecclestone dampened hopes of more races being held in the United States, at least in the near term, despite the sport's takeover by Liberty Media.
"I think it will be difficult to get more races," the Briton, who turns 86 next week, told Reuters.
"I tried in New York. The trouble with the Americans is (that) you want to do a deal with them and they want guaranteed profit before they start. I said if I knew that was going to happen, I wouldn't need you."
The sport's financial structure means promoters have to pay hefty sanctioning fees - at least $23 million in the case of the Circuit of the Americas - with ticket sales the major source of revenue.
The United States has one round of the current 21-race championship, at Austin's Circuit of the Americas, after a series of failed attempts elsewhere.
There has been regular talk of adding races on the east and west coasts and expectations have been fuelled by last month's takeover of the cash-generating sport by the company controlled by U.S. cable TV mogul John Malone.
Liberty has acquired an initial 18.7 percent stake from controlling shareholder CVC Capital Partners and plans to complete a cash and shares deal by the first quarter of 2017.
Liberty Media CEO Greg Maffei has spoken of Las Vegas and Miami as possible future venues "for the long term".
Ecclestone, who has for months been talking to would-be promoters of a Las Vegas race, said there had been a lot of meetings and joked he should have secured the concession to provide the mineral water.
"They are professional meeting people, the Americans, aren't they? They love meetings. Seven in the morning," he said.
Autoweek magazine reported in July that entrepreneur Farid Shidfar had an agreement in principle with Chinese investors to fund a Las Vegas race, but Ecclestone saw little progress.
"They are still behind him. You want them up front," he said. "That's the problem. They came over to see me last week and he called me back, must have been Monday, with another story."
Austin's place on the calendar looked in doubt last year, due to the threat of reduced state funding, but that issue was resolved and there is talk of a 120,000-strong crowd this year.
The numbers will be boosted by 10-times Grammy award winner Taylor Swift holding a concert at the track on Saturday.
"It's not the sort of contract I like produced. But we've done a lot to help them along because I more or less talked them into it in the first place," Ecclestone said of Austin. "So we'll try and keep them going."